Field of the Invention
The invention relates to a method of checking the serviceability of a tank venting system for a motor vehicle which intercepts fuel vapors and feeds them to an internal combustion engine, for a motor vehicle, the system being based on a negative pressure that is produced in the tank venting system. The monitoring system includes a container which absorbs fuel vapors and which communicates through a venting line with a fuel tank and through a regeneration line with an intake pipe of the internal combustion engine; the container has an air-admittance line that communicates with the atmosphere and which can be closed with a shut-off valve for the purpose of checking the tank venting system. The system further includes a pressure sensor that detects the system pressure of the tank venting system, a tank venting valve disposed in the regeneration line and is opened in order to feed the fuel vapor stored in the container and in order to build up a negative pressure in the tank venting system. The tank venting system is thereby classified as not serviceable at that time if the system pressure does not satisfy a predefined condition when the negative pressure is building up with the tank venting valve open and the shut-off valve closed, or the system pressure does not satisfy a further predefined condition when the negative pressure is decaying with the tank venting valve closed and the shut-off valve closed and, in addition, operating variables of the vehicle, including the internal combustion engine and the tank venting system, are checked and the process is aborted if predefined operating variable values, at which a reliable statement about the serviceability is possible, are not reached.
Such a method is described in commonly assigned U.S. Pat. No. 5,572,981 to Pfleger et al. (German published application DE 44 27 688 A1).
There, a tank venting system for a motor vehicle is checked for its functionality with the aid of a vacuum (negative pressure relative to the atmospheric condition) that is produced in the tank venting system. To this end, the prior art tank venting system includes the following features:
a container which absorbs fuel vapors and is connected via a venting line to a fuel tank and via a regeneration line to an intake pipe of the internal combustion engine, and PA1 the container having an air-intake line connected to the atmosphere that can be closed by means of a shut-off valve in order to check the tank venting system; PA1 a pressure sensor that detects the system pressure of the tank venting system; PA1 a tank venting valve which is arranged in the regeneration line and is opened in order to feed the fuel vapor stored in the container and in order to build up a negative pressure in the tank venting system; PA1 the tank venting system is classified as not serviceable at that time if the pressure gradient lies below a threshold when the negative pressure is building up (negative pressure build-up test) or the pressure gradient lies above a further threshold when the negative pressure is decaying (negative pressure decaying test) and, in addition, PA1 operating variables of the vehicle, including the internal combustion engine and the tank venting system, are checked and the method is in each case aborted if predefined operating variable values, at which a reliable statement about the serviceability is possible, are not reached. While the entire method is being carried out, the dynamic behavior of the pressure variation in the tank venting system is also monitored, for which purpose chronologically successive pressure values are registered, the average of the two pressure values is formed from these and the method is aborted if the magnitude of the difference between the average and the current pressure value lies outside a predefined dynamic range. PA1 a container for absorbing fuel vapors communicating via a venting line with a fuel tank and via a regeneration line with an intake pipe of an internal combustion engine, and the container having an air intake connected to atmosphere and which is closeable by means of a shut-off valve for checking the tank venting system; PA1 a pressure sensor detecting a system pressure of the tank venting system; PA1 a tank venting valve in the regeneration line, the tank venting valve being selectively opened for feeding the fuel vapor stored in the container and for build up a negative pressure in the tank venting system; PA1 temporarily classifying the tank venting system as not serviceable if PA1 checking operating variables of the motor vehicle, including operating variables of the internal combustion engine and the tank venting system; and PA1 aborting the diagnosis if predefined operating variable values are not attained at which a reliable statement about a functional operability of the system is possible; PA1 registering chronologically successive pressure values and using the successive pressure values as input variables for a physical model which simulates a pressure variation in the event of a leak in the tank venting system based on a gas mass flow flowing through an opening, and forming a parameter with the physical model which describes a curve variation of the pressure during the diagnosis and which contains information about a leak area; PA1 comparing a leak area with a given threshold value; and evaluating a tightness of the tank venting system on the basis of a result obtained in the comparing step. PA1 A=actual cross section; PA1 T=temperature of the gas volume; PA1 T.sub.0 =standard temperature; PA1 .alpha.=throttling coefficient; PA1 V=gas volume; PA1 P.sub.u =ambient pressure; PA1 P.sub.0 =standard pressure; PA1 P.sub.0,air =density of air under standard conditions; PA1 P.sub.0,mix =density of the fuel vapor under standard conditions; PA1 N=number of sampling steps; PA1 n=current sampling step; PA1 T.sub.A =sampling time. PA1 comparing the correction parameter with a first threshold value and aborting the method due to excessively severe gassing out of fuel, if the correction parameter lies above the first threshold value; PA1 otherwise comparing the correction parameter with a second threshold value and aborting the method and indicating an incompletely closed tank venting valve if the correction parameter lies below the second threshold value; and PA1 storing the correction parameter for further processing if the correction parameter lies above the second threshold value. PA1 forming an effective diagnostic parameter from a diagnostic parameter and a correction parameter; PA1 calculating an effective leak area from the diagnostic parameter; PA1 comparing the effective leak area with a predefined threshold value; and PA1 deducing that a leak is present in the tank venting system if the threshold value is exceeded. PA1 .rho..sub.0,air =density of air under standard conditions; PA1 .rho..sub.0,mix =density of the fuel vapor under standard conditions; PA1 T.sub.0 =standard temperature; and PA1 .rho..sub.0 =standard pressure. PA1 a degree of loading defined by a proportion of volatile fuel in an active carbon filter in the container is ascertained as an operating variable; PA1 the tank venting valve and the shut-off valve are opened for a time which depends on the degree of loading, and a flushing operation is carried out; PA1 subsequently to the flushing operation and during a predetermined interval, a maximum pressure and a minimum pressure in the tank venting system are registered; and PA1 the process is aborted if a difference between the maximum pressure and the minimum pressure exceeds a predefined limit value. PA1 opening the tank venting valve for a predefined time; PA1 checking whether, within the predefined time, the pressure in the tank venting system has reached a diagnostic negative pressure value, starting from a starting value, and if the condition in the checking step is satisfied and no infringement has occurred during the predefined time of a lambda controller threshold in a lambda control device of the internal combustion engine, abruptly closing the tank venting valve. If, on the other hand, the lambda controller threshold is infringed, the tank venting valve is closed step by step so as to avoid a sudden weakening of a fuel/air mixture fed to the internal combustion engine, and the process is aborted.